Jet deposition is a well-known technique for use in printing individual jobs or short print runs; printing heads direct droplets of liquid ink usually under the control of a computer, at a substrate. As used herein, the term “jet deposition” of a composition is intended to mean the dropwise deposition of the composition onto a predetermined location on a substrate to build up a two dimensional image or a three dimensional object in accordance with data stored in a computer file, e.g. a CAD design.
Early jet deposition heads were based on bubblejet technology and were limited to using aqueous inks. Aqueous inks require time to dry and this factor limits their application; also, the need to provide for space where the drying takes place limits the maximum rate at which printing can take place.
More recently piezoelectric jet printer heads have become commercially available and allow the use of photo or heat curable organic inks. It is therefore possible to cure a deposit immediately after it is formed, e.g. using a UV lamp to bring about photocuring. This eliminates the need to dry the ink and so increases production speed in conventional two dimensional printing. Curing also allows the use of jet deposition to manufacture three dimensional articles since it is possible to deposit droplets in successive layers to build up an object of a desired shape under the control of a computer program. Jet deposition has substantial advantages in being able to produce a wide variety of differently shaped deposits by changing the controlling computer file and so is relatively quick and cheap to set up and to switch from one image or object to another. It is therefore well suited for on-demand printing of single articles or short print runs.
Jetting successive layers to build up three dimensional objects is useful in the production of prototypes during product development or for short run production since it is possible to produce three dimensional objects more rapidly using these techniques as compared to known techniques, for example injection moulding where the manufacture of the mould is highly time consuming.
Recent techniques such as micro-spraying also allow the use of curable organic liquids to form two dimensional images and three dimensional articles; micro-spraying produces directed sub-droplets formed from directed droplets that are subjected in flight to acoustic waves which break up the droplets into even finer sub-droplets, see for example U.S. Pat. No. 6,182,907 (Mitsubishi Corporation) and U.S. Pat. No. 6,309,047 (Xerox Corporation).
However, there are several constraints on the types of compositions that can be used in jet deposition. One requirement is that the surface tension should be less than about 40 dynes/cm. A further important requirement is that, at deposition temperature, the composition should have a viscosity of approximately 2-30 cps. Unfortunately many curable compositions that can provide desirable final properties also have high viscosities and are therefore difficult to jet. There have been several proposals to reduce the viscosity of curable compositions to allow them to be jetted.
The viscosity of the composition can in some instances be reduced to the above range by jet deposition at temperatures above room temperature, for example temperatures up to and over 100° C., e.g. 65-75° C. are straightforward to use. Indeed, temperatures of up to 180° C. are possible with certain technologies, e.g. PolymerJet™ available from MicroFab Technologies Inc., Plano, Tex., U.S.A.
It has also been proposed to deposit phase change compositions, e.g. waxes, that are liquid at a jetting temperature (e.g. 65-75° C.) but solidify at room temperature. In this way three dimensional deposits have been built up. However, wax is not a robust material and the phase change inks have very limited application. There have been proposals to incorporate a curable material into a wax (see for example U.S. Pat. No. 5,855,836), but this is of only limited benefit.
Another proposal to reduce the viscosity of jettable curable compositions is to add a volatile diluent to the composition. However, the use of volatile solvents that evaporate after deposition is not desirable for health and safety reasons since the solvent gives rise to a fire hazard and can be damaging to the health of anyone inhaling substantial quantities of the solvent. Formulations involving solvents also do not yield the required final properties.
Acrylic compositions can be made having a low viscosity and are radiation curable and they have been widely investigated and are becoming increasingly common in the graphics art industry due to their rapid drying and solvent-free properties. Despite the low viscosity and fast curing rate of acrylate-based inks the bulk properties of these inks are generally unsuitable for many applications, including the building up three dimensional objects, since acrylate polymers are brittle and shrink on curing. In addition, low viscosity acrylates can be irritants and therefore need careful handling.
WO00/23521 discloses a compounded polyester resin including both a high and low viscosity polyester. EP0848949 discloses a stereolithography composition containing an oxetane, an epoxy and a hydroxy-containing compound, e.g. tetrahydrofuran. However, neither specification discloses the use of the composition for jetting.
U.S. Pat. No. 4,195,148 discloses a method of preparing polyurethane using a lactone as a viscosity modifier. GB-2011931 discloses a water-soluble paint binder comprising a polyester resin modified with a polysiloxane and a carboxy group-containing film-forming resin.
EP0375332 discloses a mixture of a high viscosity methyl cellulose and a low viscosity methyl cellulose, which is useful as a binder and thickener in adhesive compositions.
DE19534327 discloses an aqueous composition for coating paper. U.S. Pat. No. 4,986,850, WO00/34400, EP0071345, U.S. Pat. No. 5,889,084 disclose jettable printing inks that contain water or a volatile solvent.
JP-A-2002 317139 discloses an oxetane-based radiation-curable ink for ink jet printing under humid conditions. JP-A-2001 220526 describes a jettable ink containing compounds having an oxetane ring.
WO99/29788 (Xaar Technology Ltd) discloses a radiation curable jettable printing ink that includes a mixture of mono-, di- and tri-functional acrylates for printing on paper and plastics.
There is a continuing and increasing need to provide a composition that can be rapidly cured and that avoids the brittleness and shrinkage problems of acrylate resins and so allows jet deposition to be used for making multilayer deposits, for example to make three dimensional objects by building up successive layers or to be used in printing, e.g. on demand in single copies or in short print runs, books, carpets, wall papers, large format images (e.g. posters and advertising hoardings) and floor tiles and other applications where scratch-resistant, tough, permanent, chemical resistant printed images are desired. The printed image is preferably of uniform thickness so that no part of the image stands proud of the rest of the image. Such jetting applications are within the scope of the present invention.
Curable compositions can also be used for the rapid manufacture of three dimensional objects since rapid curing allows the next layer to be deposited almost immediately afterwards to build up a three dimensional article (see e.g. WO01/68375, U.S. Pat. No. 6,259,962, WO00/52624 and WO00/76772). In producing three dimensional objects, it is important that the material from which the object is formed is highly cohesive since otherwise the object will fall apart and have little or no tensile strength. Thus individual droplets used to form the object should cohere to one another and, indeed optimally, it is desirable that the droplets should be joined seamlessly, i.e. the individual droplets are not discernible, in the final three dimensional object.
UV curable resins are well known in the field of stereolithography to make three-dimensional objects but the requirements for stereolithographic compositions are very different than those for application by jetting and also the curing environment is different in stereolithography as compared to jetting.
The present invention provides a jettable composition having improved combination of properties, especially a good combination of tensile strength and ductility, e.g. elongation at break and tensile modulus, while allowing a broad range of polymers and resins to be used, since the composition provide cohesion between adjacent deposited droplets.